Composite materials, for example in the form of or including combinations of glass, coal, or aramid fibres in a binder typically made from polyester, epoxy or polyurethane, are constantly gaining ground as load-bearing structures.
In relation to metallic materials, composite materials often have a lower modulus of elasticity and are therefore subjected to a larger deflection under load conditions otherwise the same.
Angle sections have several favourable qualities which have led to their being used in load-bearing structures of various kinds. Angle sections have relatively good bending stiffness in two planes and they are easy to join together, for example by means of bolts, because there is good access to the flanges of the angle section. One of the main weaknesses of the angle section is its relatively poor torsional rigidity, which, in many structures, calls for the use of other profiles. Alternatively, a more complicated design of the structure may remedy the weakness.
Hollow profiles have a relatively good torsional rigidity both when made as circular pipes and when made as rectangular pipes. However, it is relatively complicated to bolt pipes, even rectangular pipes, together because the access to, for example, nuts that are inside the pipe may be conditional on a larger access hole being drilled into the opposite side of the pipe. Sometimes spacer sleeves are used inside the pipe when through bolts are used in the pipe. Both embodiments weaken the load-carrying capacity of the pipe.
Composite profiles are often made with a predominant portion of longitudinal fibres. Therefore they have relatively low tearing strength near their end portions.